There are many applications for display systems embodying cathode ray tubes (CRT) which are driven by digitally controlled video. Typically, a computer may provide three bits of digital control with 000 corresponding to the background, 111 corresponding to the brightest shade, and 001 to 110 corresponding to six gray shades in between. From a subjective visual consideration, it is very desirable that all of the consecutive gray shade steps exhibit the same contrast. In other words, it is important that the eye distinguish between gray shade levels graded uniformly. Subjectively, equal changes in contrast require equal ratios of luminance. In computer driven display systems, .sqroot.2 is a typical ratio because it provides adequately distinguishable gray shades. Using .sqroot.2 as an example, that means that if a given level, L.sub.i, has a luminance of 1.0 foot-Lamberts (FT-L), the next brighter level, L.sub.i+1, would have a luminance of 1.4 FT-L. Also, L.sub.i+2 would have a luminance of 2.0 FT-L, etc.
Generally, the digital code is converted to an analog signal that is used as a drive voltage for the cathode of the CRT and a negative bias voltage is supplied to the grid. In general, a typical CRT transfer characteristic of cathode voltage versus brightness is such that for constant input changes in drive voltage, the changes in luminance are not at a fixed ratio. That is, the transfer characteristic is not ideally logarithmic. More specifically, the cathode current of a CRT may usually be expressed by the equation EQU I.sub.C =kV.sub.d.sup..psi. V.sub.C.sup..epsilon.
where k is a CRT system constant and is often called the "modulation constant;" V.sub.d is the drive voltage; V.sub.C is the cut-off voltage; .psi. is an exponent often called the "gamma" of the modulation characteristic; and .epsilon. is an exponent (.psi.+.epsilon.=3/2). Furthermore, with a typical CRT, brightness is not simply related to cathode current. Rather, at high beam currents, aperture losses increase due to beam-bundle spread. Also, at high brightness, the phosphor efficiency decreases due to saturation effects. Thus, in a given system, the transfer characteristic of a CRT is typically quite non-linear and not defined with adequate accuracy by a simple law. It follows that there is a problem converting the digital codes to analog signals that provide equal changes in contrast between the consecutive gray shade steps.
The problem is made much more complex by the fact that in most operational systems, it is desirable to adjust a brightness control so that the tube luminance is optimized for the ambient light level of the environment. As the brightness is adjusted, which is commonly provided by changing the direct current bias to the grid, the operating window of gray shade levels on the transfer characteristic is moved along the non-linear curve. Accordingly, to maintain equal changes in contrast between the consecutive gray shade steps as the brightness is adjusted, the ratios of luminance for consecutive gray shade steps must be fixed over the entire operating curve rather than just a portion of it. The reason that it is desirable to adjust the brightness to be optimum for the ambient light level of the environment is that if the display is too bright with respect to the operational room, increased glare causes operator eye strain decreasing efficiency; also, if the display is too dim with respect to the room, the operator's visual adaptive time after looking around the room is greatly increased. It has been found that operator errors increase as the display becomes too bright or too dim with respect to the brightness of the room. Due to practical considerations, the ambient light level of many operational rooms varies through the course of a day and with functional usage resulting in the desirability of brightness adjustments.